1. Field of the Invention
This invention relates to convulcanizable compositions based on fluoroelastomers.
More in particular the present invention relates to covulconizable compositions of fluoroelastomers having an improved chemical stability.
2. The prior art
The fluoroelastomers based on vinylidene fluorides vulcanized according tot he bisphenol or dihydroxyl system (Kirk Othemer, Encyclopia of Chemical Technology, vol. 8, page 506, 1979) are excellent produce which are utiliezed in the field of industry, in particular in the sectors of motorvehicles, of oil and of energy in general.
In the automotice sector, the fluoroelastomers are chiefly utilized in the fuel circulation system or as components of parts of the motor and of parts of the grearbox-differential-driveshaft aggregate.
In the first case there may be cited, as examples of appliances, diaphragms for feed pumps, valve seats, check valves, flexible hoses and in general sealing gaskets for the fuel.
In the second case, analogous examples are; gaskets for gears, sealing jackets for cylinders in diesel motors, gasket for the crankshaft, check valves for the exhaust gases, gaskets such as oil seal rings and the like.
In the oil and energetic field the fluroelastomers are utilized for appliances in direct contact with the crude oil, chiefly at high temperatures and pressures, such as e.g. for seals, valves and pumps or parts thereof.
In this field, an important appliance for the fluoroelastomers is represented by the expansion joints used in the discharge lines of flue gases in the thermoelectrical stations, where a high resistance to high temperatures in combination with the corrosive action of the acid gases is required.
In these sectors, as well as in others, such as the electrical and electronic onse, the fluoroelastomers are utilized because of their properties of high thermal stability and chemical resistance even at high temperatures.
In the last years, however, and in a few types of appliances, particular technological requirements have arisen, which cannot be met by the present manufactured articles, vulcanized according to the bisphenol or dihydroxyl systems.
Particularly involved is the chemical stability also under very severe operating temperature conditions in the presence of vapors, of mineral acids, of lubricating fluids characterized by a high aggressivity, such as the oils of series SF additioned with compounds which exhibit a high aggressivity toward the fluoroelastomers based on vinylidene fluoride.
Furthermore, a general improvement of the processability characteristics of the mixes is required, in order to permit the molding of articles such as oil seal rings, and the possibility of easily extruding, even with complex profiles, the mixes for obtaining suitable preforms.
The problem of the chemical stability has been solved by the use of the peroxide vucanization.
This vucanization, however, does not meet the abovementioned processability requirements, in particular as regards molding and extrusion; furthermore, the processability itself proves to be less sure (Rubber Chemistry and Technology, vol. 55, page 906, 1982), and also the compression set test is much poorer (Kirk-Othmer, Encyclopedia of Chemical Technology, vol. 8 page 510, 1979).
In European Paten Application No. 84.110707 there were also prepared and described covulcanized compositions which permitted to achieve certain improvements, which nevertheless exhibit, under certain conditions, not yet optimum chemical-thermal and mechanical properties.